Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 100

Commercial cellular technologies before 5G depended critically on spatial configuration and the separation of downlink (DL) and uplink (UL) frequencies. Frequency allocation to base stations ensured that interference from other base stations is minimal. In spread spectrum systems such as 3G and LTE, the base station assigns different spreading codes to the different end users to mitigate interference on the UL. Mobile end users can hand over from one base station to another while switching to a different frequency and after obtaining a different spreading code from the new base station. Figure 6.4 shows an oversimplified frequency planning illustration for a pre‐5G cell tower deployment where seven frequency bands can be reused spatially to create full spectrum coverage. Factors such as terrain and weather can change this plan and cellular providers rely on comprehensive testing of the cellular infrastructure performance to change the frequency allocation of such a simple plan where areas that have more dense end users can use more base stations with more frequency bands to add capacity where demand is higher.⁵

Figure 6.4 Traditional cellular frequency spatial separation planning.

With 5G and the dense deployment of different size cells, a received signal is impacted by the distance between many transmit and receive pairs using the same frequency. 5G includes overlay of different technologies and different access points, which have different areas of coverage that can overlap. Cellular 5G is heterogeneous in many aspects, including the following:

1 The deployment of different cell types, as shown in Table 6.1. Each cell type can have a different area of coverage and these areas can intersect and be overlaid on top of each other.

2 The mixed use of FD links, with directionalities to increase spectrum reuse, with LTE links that separate the uplink from the downlink channels.

3 The opportunistic use of available spectrum mixed with the use of provisioned spectrum.

4 The mix of unplanned deployment of 5G cells, which may or may not have fiber connectivity to the core network, with LTE fixed infrastructure.

5 The ability to operate in a very wide range of frequency bands spanning from below 6 GHz to 102.2 GHz.

In essence, cellular 5G provides high capacity access through randomly located nodes (end users and cells), irregular infrastructure, and dynamic spatial configurations. The cellular 5G paradigm is a major shift from previous cellular technologies that require the use of different spatial models.

The impact of the distance between the transmitter and the receiver on signal power has been studied with different propagation models. Wireless systems have long been designed based on link‐budget analysis, fading margins, and the ability to tradeoff range for transmission rate. The 5G paradigm requires transmit and receive node pairs to continually consider the timely use of a frequency in light of spatial separation to avoid excessive interference. In the multidimensional spectrum sensing model presented in the previous chapters, space becomes the most challenging dimension to model with cellular 5G. While time and frequency separation is easier to model, space modeling encounters the leakage of undesired signals and the impact of co‐site interference in addition to the continual change in the transmitting and the receiving nodes locations. 5G has limited practical options to reduce interference keeping in mind that reducing signal power would reduce the signal to interference ratio (SIR)⁶ while increasing signal power will reduce the chance of spectrum reusability.